1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or C-MOS sensor used in a compact image pickup device and more particularly to imaging lenses which are built in image pickup devices mounted in increasingly sophisticated mobile terminals such as smart phones, mobile phones, and PDAs (Personal Digital Assistants), and game consoles and information terminals such as PCs.
2. Description of the Related Art
In recent years, the market of tablet multifunctional terminals as typified by smart phones has been rapidly expanding and there is a general trend that such terminals incorporate high-performance, high-quality cameras which cope with an increase in the number of pixels, for example, over 8 megapixels. Since the trend towards cameras which cope with an increase in the number of pixels is expected to be accelerated, the imaging lenses built in image pickup devices are required to provide high performance to cope with an increase in the number of pixels and also be compact enough to suit low-profile product designs. Furthermore, since pixel size becomes smaller with the tendency toward smaller image sensors and a larger number of pixels, the imaging lenses are strongly expected to provide high brightness.
In addition to many types of imaging lens composed of four lenses (four-element imaging lens) which have been proposed so far, imaging lenses composed of five or six lenses (five-element or six-element imaging lens) are proposed in order to meet the trend toward higher performance and more compactness.
For example, JP-A-2007-264180 (Patent Document 1) discloses an imaging lens which includes, in order from an object side, a positive first lens having a convex object-side surface, a negative meniscus second lens having a concave image-side surface, a positive meniscus third lens having a convex image-side surface, a negative fourth lens as a double-sided aspheric lens having a concave image-side surface near an optical axis, and a positive or negative fifth lens as a double-sided aspheric lens.
Also, JP-A-2011-085733 (Patent Document 2) discloses an imaging lens system which includes, in order from an object side, a first lens group including a first lens with a convex object-side surface, a second lens group including a second lens with a concave image-side surface, a third lens group including a meniscus third lens with a concave object-side surface, a fourth lens group including a meniscus fourth lens with a concave object-side surface, and a fifth lens group including a meniscus fifth lens with an aspheric object-side surface having an inflection point, and also discloses an imaging lens composed of six lenses including a positive lens having a slightly convex surface on the object side of the above lens system and a slightly concave surface on its image side.
The imaging lens described in Patent Document 1, composed of five lenses, realizes a high-performance imaging lens system which corrects axial chromatic aberrations and chromatic aberrations of magnification and copes with an increase in the number of pixels, by optimizing the lens material and lens surface shapes. However, its total track length is as long as about 8 mm and there is difficulty in applying it to an increasingly low-profile image pickup device. Also, the F-value is about 2.8, which is not sufficient to provide high brightness as expected in recent years.
Patent Document 2 discloses a high-resolution imaging lens composed of five lenses and a high-resolution imaging lens composed of six lenses. The total track length of the five-element imaging lens is about 6 mm and that of the six-element imaging lens is about 6.6 mm, so these imaging lenses are considered to meet the demand for a low-profile design to some extent. However, in this technique, the F-value is about 2.8, so it is difficult to provide both high resolution and high brightness as expected in recent years.